1. Field of the Invention
The present invention relates to semi-conductor wafer handling equipment, and more particularly, to a removable handle for semi-conductor wafer processing cassettes.
2. History of the Prior Art
In the manufacture of semi-conductor devices, such as integrated circuits, a plurality of devices are formed on a single circular wafer of silicon material. The wafer is typically circular and on the order of 6 inches in diameter. The wafers are put through a number of sequential processing steps, including coating them with photo-resists, exposing them to the optical patterns formed on photo masks, and passing them through both liquid and gaseous treating environments.
The processing of a silicon wafer containing a plurality of semi-conductor devices requires a high degree of cleanliness and sterility in the environment in order to produce acceptable devices. The ability of a semi-conductor device to perform satisfactorily from both an electrical and mechanical standpoint depends on the nature and quality of the materials forming the various layers of the device. The chemical composition of these materials must be extremely pure. The introduction of any foreign matter into the environment where the wafers are being processed results in a decrease in the "yield" of the wafer. The yield is the number of devices that can pass the required electrical tests of the device after the processing has been completed. This is usually expressed as a fraction of the total number of devices processed on the wafer. Thus, the higher the purity of the processing environment and processing techniques used in manufacturing the semi-conductor devices, the greater the yield and hence the greater the financial return to the manufacturer.
Silicon wafers are generally handled in inert plastic frames containing a plurality of vertically arranged dividers. Each divider defines a pocket to receive a silicon wafer and holds it securely in a vertical orientation while isolating it from adjacent wafers. The plastic frame and divider assembly is generally referred to as a "cassette." A typical industrial cassette will hold on the order of 60 wafers for processing. The cassettes must be physically moved by operators from processing station to processing station where the cassettes are placed into indexing mechanisms forming part of the processing machinery of each station. The wafers are automatically removed and again placed into cassettes by the indexing mechanisms after the processing step at each station has been completed. To guard against the introduction of impurities into the silicon wafer processing environment, the operators all wear caps, gowns and surgically sterile rubber gloves when they are handling the cassettes. In addition, the operators frequently change gloves, sometimes three to four times per day, to reduce the amount of contaminants effecting the wafer processing environment. The manner in which even gloved operators handle the cassettes has been shown to have a substantial impact upon the number of unwanted particles found contaminating the surface of the wafer. This, of course, results in impurities and defective devices on the wafer. It has also been noted that each individual operator seems to have a fairly consistent yield rate which is apparently somehow connected with the manner in which that operator handles the wafer cassettes.
It has been shown by tests that the contact of the wafer cassette by a surgically gloved human hand also produces a certain level of impurities in the wafers closest to the contact. In order to avoid such contact, various cassette handling tools have been proposed within the industry including elongate single handles, latching tongs, and other mechanisms for increasing the minimum distance between the operator and the cassette. However, there are a number of inherent disadvantages in the prior art wafer cassette handling tools. Many of the prior art tools require two handed operation and thereby decrease operator dexterity. Such tools prevent the operator from having one hand free to provide stability and backup in the event it is needed. Additionally, the two handed operation doubles the risk of contamination due to the close location of two hands to the wafers. A single cassette full of finished silicon wafers may be worth on the order of $25,000.00 and as such must be handled cleanly, carefully and securely. Other proposed cassette handles have included loose fitting devices which temporarily engage the cassette for handling. These devices are subject to slippage, droppage and disconnection due to the non-rigid attachment of the handle to the cassette.
It would thus be desirable to provide an easy to use removable handle for a wafer cassette which can be selectively and rigidly attached to the cassette for as long as necessary in the processing operation. The handle should be quickly detachable from the cassette for reuse with another unit. The detachable handle of the present invention provides such a device.